1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly, to a laser diode capable of reducing strain by preventing overflow of carriers caused by injection of a number of carriers in a semiconductor light emitting device using GaN.
2. Description of the Related Art
Referring to FIG. 1, a semiconductor light emitting device comprising a conventional semiconductor laser diode or a light amplifier, consists of active layers, InyGa1-yN/InyGa1-yN layers 1 and 2, a p-AlxGa1-xN carrier blocking layer 3 and a p, n-GaN light waveguide layer, having a multi-quantum well (hereinafter, referred to as a MQW) structure.
FIG. 2 is an energy band diagram of the structure of the semiconductor light emitting device of FIG. 1. In this figure, carriers having the MQW structure are formed so as not to pass through an energy barrier of a carrier blocking layer.
A semiconductor light emitting device such as a bluish green laser diode or a light amplifier must be equipped with such an energy band structure. The bluish green laser diode is an important element required for recording on and reproducing data from a high-density recording medium such as a DVD.
In order to realize a bluish green laser device which usually operates at a low laser-emission starting current and has a good temperature property, the bluish green laser device should be designed to have a structure which enables efficient current injection into an active layer and the reduction of the number of carriers passing through the active layer. In general, to prevent carriers from overflowing, a method for inserting an electron blocking layer 3, illustrated in FIG. 1, which can block electrons, that is, carriers, has been used.
In the case of a GaN bluish green laser basically made of a GaN compound semiconductor, there is no substrate whose lattice constant is consistent with that of GaN, and thus, the quality of crystals such as GaN, InGaN, and AlGaN grown by using a MOCVD method, is not good. Accordingly, light gain of InGaN used as an active layer is reduced. Therefore, a very large amount of injection current, that is, a large number of carriers is needed for generating a laser beam. Moreover, as the amount of injected current increases, the number of carriers overflowing into not only the active layer but also a GaN layer or an AlGaN layer used as a barrier also increases, and thus, it is impossible to efficiently generate a laser beam. To solve this problem, as illustrated in FIG. 1, insertion of a carrier blocking layer consisting of an AlGaN mono layer having a thickness of 200 xc3x85 has been widely applied by various research groups. However, the insertion of an AlGaN mono layer can hardly prevent overflow of carriers having an energy higher than a barrier. To block high energy carriers, a layer formed of a compound containing Ga, N and a large amount of Al may be used. However, the AlGaN layer causes additional strain with respect to a GaN layer whose lattice constant is not consistent with that of the AlGaN layer, whereby cracks in samples usually occur. Also, in order to efficiently block carriers, a p-type impurity doping process may be performed. However, it is well known that a process for doping the layer formed of a compound containing Ga, N and a large amount of Al with a p-type impurity is very difficult.
To solve the above problems, it is an object of the present invention to provide a nitride semiconductor light emitting device capable of efficiently preventing current injected into an active layer from overflowing a barrier and minimizing strain.
It is another object of the present invention to provide a nitride semiconductor light emitting device capable of efficiently preventing current injected into an active layer from overflowing a barrier, minimizing strain, reducing the amount of optical loss in the direction of installation of a substrate, preventing deterioration of an active layer and simplifying a process for manufacturing the nitride semiconductor light emitting device.
Accordingly, to achieve the first object of the present invention, there is provided a nitride semiconductor light emitting device including an active layer formed of a GaN family compound semiconductor and multi-quantum barrier layers formed by repeatedly depositing a double layer consisting of an AlxGa1-xN layer (0 less than x less than 1) and a GaN layer at least two times, at either the upper or lower side of the active layer, by which an energy band has a multi-quantum barrier structure. The nitride semiconductor light emitting device further includes GaN light waveguide layers formed at the upper and lower sides of the active layer or at the upper and lower sides of the multi-quantum barrier layers. The active layer is formed by depositing a double layer consisting of an InxGa1-xN layer and an AlyGa1-yN layer, a double layer consisting of an InxGa1-xN layer and an InyAlzGa1-y-zN layer, a double layer consisting of an InxAsyGa1-x-yN layer and InzGa1-zN layer or a double layer consisting of an InxAsyGa1-x-yN layer and an AlyGa1-yN layer a predetermined number of times to form a multi-quantum well structure and at this time, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6y less than 1, 0xe2x89xa6z less than 1, x+y less than 1 and y+z less than 1.
Preferably, the multi-quantum barrier layer is formed of double layers of the AlxGa1-xN layer and the GaN layer by making the thickness of the AlxGa1-xN layer of each double layer differ from the thicknesses of the AlxGa1-xN layers of the other double layers or making the thickness of the GaN layer of each double layer differ from the thicknesses of the GaN layers of the other double layers, thereby making the energy levels of multi-quantum barrier layers differ from each other. Or, it is preferable that the multi-quantum barrier layer consisting of the AlxGa1-xN layer and the GaN layer, is formed by making the value of x for of aluminum of the AlxGa1-xN layer of each double layer differ from the value of x for the AlxGa1-xN layers of the other double layers, thereby making the energy levels of multi-quantum barrier layers differ from each other.
Preferably, the multi-quantum barrier layer is formed by repeatedly depositing a double layer consisting of an AlxGa1-xN layer and an InyGa1-yN layer (0 less than x less than 1, 0 less than yxe2x89xa61), so that its energy band can have a multi-quantum barrier structure. In this case, the multi-quantum barrier layer is formed by making the thickness of the AlxGa1-xN layer of each double layer differ from the thicknesses of the AlxGa1-xN layers of the other double layers of making the thickness of the InyGa1-yN layer of each double layer differ from the thicknesses of the InyGa1-yN layers of the other double layers, thereby making the energy levels of multi-quantum barrier layers differ from each other. Or, the multi-quantum barrier layer is formed by making the value of x for the AlxGa1-xN layer of each double layer differ from the value of x of aluminum of the AlxGa1-xN layers of the other double layers, thereby making the energy levels of multi-quantum barrier layers differ from each other.
To achieve the second object of the present invention, there is provided a nitride semiconductor light emitting device in which, among p-type material layers formed at one side of an active layer and n-type material layers formed at the other side of the active layer, a p-type clad layer is removed, an n-type clad layer is formed to have an energy band thicker than the counterpart of the prior art, and the carrier-blocking efficiency of a carrier barrier layer formed between the p-type material layers and the active layer is enhanced.
Also, to achieve the second object of the present invention, there is provided a nitride semiconductor light emitting device including a substrate, an active layer formed on the substrate, in which light emission occurs, an n-type material layer for generating a laser beam which is formed between the substrate and the active layer and includes an n-type clad layer for preventing light loss in the direction of installation of the substrate, a carrier blocking layer, a p-type waveguide layer and a p-type compound semiconductor layer which are sequentially deposited on the active layer and an n-type electrode and a p-type electrode generating a potential difference for diffusion of electrons to the active layer. Here, the n-type material layer comprises an n-type waveguide layer formed between the n-type clad layer and the active layer and an n-type compound semiconductor layer formed between the n-type clad layer and the substrate and connected to the n-type electrode. The active layer is a III-V group nitride compound semiconductor layer having a multi-quantum well structure.